When we last left Project Sti we had done the first round of engine mods and reached the limits of the stock fuel system. Project STi was running on the verge of dangerous leanness with the air/fuel ratio fluctuating and the power levels inconsistent from pull to pull. If one was concerned about engine life, this condition was barely acceptable for street use and defiantly suicidal on a road course.
Eric Hsu of XS Engineering informed us that typically the stock STi fuel system is marginal even with our bolt-on engine mods. Eric said that the stock fuel pump and injectors are too small for anything other than basic bolt-ons. To resolve this issue we used a high volume fuel pump kit from Powertech Imports. The kit consists of a 255-liter-per-hour Walburo fuel pump with Subaru compatible pump bibs and pick up. The kit installed in minutes, practically dropping right into the stock fuel tank. The high volume pump is slightly noisier than stock, but the difference is hardly noticeable. The pump should provide enough fuel to reach all of our immediate power goals for Project STi.
We also upgraded the stock injectors at this time. The stock STi injectors flow 550cc/min. XS Engineering modifies the stock injectors to flow 780cc/min for a nominal fee. XS checks the injectors' balance at this time as well. In order to run the larger injectors, XS recommends a reflash of the stock ECU to XS' specifications. We'll get more into the details of that later.
Another issue we were facing was the poor performance of the car's stock top-mount intercooler. When we drove the car on the track, we could feel the power drop like a rock over the course of three laps. The power loss felt like it was in the 30 hp range.
XS tested the stock intercooler on their dyno with the hood closed so the hood scoop would function, while huge fans were directed straight into the hood scoop. Eric from XS shared test data on the stock intercoolers effectiveness, and we've listed it below.
As you can see, after just 4 pulls the stock intercooler is completely ineffective. During track conditions the cooling airflow might be a little better at times than even a very powerful fan but at low speeds it will be worse. No wonder we were feeling such a large power drop. Although it would be difficult to tune the ECU more aggressively with such a widely possible variation in the intake temperature, something had to be done to address this issue.
At one time we were told by Subaru public relations that the top mount intercooler evolved from Subaru's WRC rally experience. The intercooler was top mounted to make it less prone to damage. However, if you look at any of the WRC Subarus now, you will note that they all have front mount intercoolers. XS Engineering stepped up to help us solve this problem with their Power IC front mount intercooler system made specifically for the STi.
The Power IC has a huge tube and fin core. In testing, the tube and fin heat exchanger performed better than a bar and plate element in regards to both pressure drop and cooling efficiency. The tube and fin element also proved to be much lighter. The Power IC came with lightweight polished aluminum intake tubes treated with a protective anti-corrosion coating. The tubes fit well, making the installation easy. However, we did have one issue. The Power IC's compressor discharge pipe went through the same hole in the chassis as our AEM cold air intake did, which made for a rush trip back to XS in the middle of the night to pick up one of their Powermax cold air intake systems as the AEM part no longer fit.
According to XS Engineering, the Powermax cold air intake has been dyno-tuned to be compatible with the intake tract length changes caused by the front mount intercooler's long pipes; and has been found to be the most powerful cold air intake on the market when coupled with a front mount intercooler. The Powermax intake fits into a chassis hole further forward of the hole that the AEM intake used, assuring compatibility with most front mount intercooler kits. This is good because most of the other front mount compatible air intakes are simply short ram types that gulp hot underhood air. The Powermax intake had a chrome finish that matched the intercooler piping, and set off the engine compartment nicely.
To demonstrate the front mounts' effectiveness, Eric shared some more data he had collected in prior testing of the Power IC. Under nearly the same test conditions on the same car, the following data was collected:
This is an amazing temperature increase of only 5 degrees! The additional surface area of the long intake pipes out of the engine's heat-soak zone probably resulted in lower temperature to the intercooler as well. Keeping the intake charge up to 160 degrees cooler allows for more boost, more spark advance and a leaner mixture with greater safety.
After the installation of the intercooler, we were now ready for Eric to work his tuning magic on the stock ECU. Based on his prior experience with the STi, Eric reflashed the ECU using ECUTEK's software package with XS Engineering's basic performance program. He strapped the car to the dyno with a wide band A/F ratio meter to see if more tuning would help our car. The reflash consisted of changes to the injector-sizing constant to compensate for the larger injectors, it also changes the drive by wire throttle response. By adding more spark advance and using a slightly leaner mixture, we can take advantage of the much cooler, more stable intake air temperatures than our larger front mount would give. Since intake air temps are much cooler and stable, XS was able to safely increase the boost pressure from the stock 14 psi to 20 psi.
Project STi responded to the reflash with a vengeance, and too well for the stock clutch that immediately went up in smoke. Eric aborted the testing and reduced the boost to 18 psi, at which point the stock clutch held. Project STi was pumping out 313 hp and 314 lb-ft of torque at this point. This was a gain of 28 whp over our last installment. The thing that numbers alone don't show is that the power gain is across the board from idle all the way to the fuel cut with a peak gain of 50 hp at 5000 rpm. The power is broad and the dyno chart flat with the power level hovering over 300 hp from 5000 rpm to 7700 rpm. The engine makes over 200 whp from 3500 rpm on up - this is broad, useable, maximum under-the-curve power.
Many people say that a front mount intercooler with its larger core, larger end tanks and long ducting contribute to turbo lag on a WRX/STi. XS proves this is not an issue. With proper pipe sizes and by not going overboard in core dimensions, the XS set-up actually has less lag and more bottom end power than the stock top mount intercooler. If our clutch had not begun to slip we probably would have been able to extract another 12-20 hp from the stock turbo with ease. We will revisit this issue when we upgrade our clutch and add a light flywheel later.
Our car feels like it got a new lease on life - the engine is much more snappy and the throttle response is crisp with no drive-by-wire throttle lag. The car feels eager and jumpy. There is hardly any discernable turbo lag. We can say that there seems to be no negatives associated with the front mount intercooler and reflash, a very rare thing for an aftermarket performance modification. The reflash has probably been the biggest single improvement to the car to date.